The present invention relates to digital communication and, more particularly, to a method and system for Peak-to-Mean-Envelope Power Ratio (PMEPR) reduction in multicarrier transmissions such as Orthogonal Frequency Division Multiplexing (OFDM) systems.
One approach to the design of a bandwidth-efficient communication system in the presence of channel distortion is to subdivide the available channel bandwidth into a plurality of equal-bandwidth subchannels, with the bandwidth of each subchannel being sufficiently narrow that the frequency response characteristics of the subchannels are nearly ideal. With each of n subchannels we associate a subcarrier exp(i2π(f0+lfs) (1≦l≦n) where i is the square root of −1, f0 is the carrier frequency and fs is the carrier spacing. In OFDM, the symbol rate of each of the subchannels is set equal to the separation of adjacent subcarriers so that the subcarriers are orthogonal over the symbol interval, independent of the relative phase relationships of the subcarriers.
The complex envelope of the resulting multicarrier signal is
                    m        ξ            ⁡              (        t        )              =                  ∑                  t          =          1                n            ⁢                        ξ          l                ⁢                                  ⁢                  exp          ⁡                      (                          ⅈ              ⁢                                                          ⁢              2              ⁢                              π                ⁡                                  (                                                            f                      0                                        +                                          lf                      s                                                        )                                            ⁢              t                        )                                ,      t    ∈          [              0        ,                  f          s                      -            1                              )      where ξ=(ξ1, . . . , ξn) is a complex vector with entries drawn from a constellation Q of symbols. The admissible codewords ξ constitute a code C. Defining θ=2πfst gives
                                    m          ξ                ⁡                  (          θ          )                            =                                  ∑                      l            =            1                    n                ⁢                              ξ            r                    ⁢                      exp            ⁡                          (                              ⅈθ                ⁢                                                                  ⁢                l                            )                                                  ,      θ    ∈          [              0        ,                  2          ⁢          π                    )      Then
            P      ⁢                          ⁢      M      ⁢                          ⁢      E      ⁢                          ⁢      P      ⁢                          ⁢              R        ⁡                  (          ξ          )                      =                  max                  θ          ∈                      [                          0              ,                              2                ⁢                π                                      )                              ⁢                                                                            m                ξ                            ⁡                              (                θ                )                                      2                                              E          ⁢                      {                                                          ξ                                            2                        }                                          P      ⁢                          ⁢      M      ⁢                          ⁢      E      ⁢                          ⁢      P      ⁢                          ⁢              R        ⁡                  (          C          )                      =                  max                  ξ          ∈          C                    ⁢              P        ⁢                                  ⁢        M        ⁢                                  ⁢        E        ⁢                                  ⁢        P        ⁢                                  ⁢                  R          ⁡                      (            ξ            )                              
A major problem with multicarrier modulation in general and with OFDM systems in particular is this PMEPR, the high peak-to-average power ratio that is inherent in the transmitted signal. Large signal peaks occur in the transmitted signal when the signals in the n subchannels add constructively in phase. Such large signal peaks may saturate the power amplifier at the transmitter and thus cause intermodulation distortion in the transmitted signal. Intermodulation distortion can be reduced by reducing the power in the transmitted signal, so that the power amplifier always is operated in the linear range; but such a power reduction results in inefficient operation of the OFDM system.
Various solutions to this problem have been proposed. For example, Jones et al., in U.S. Pat. No. 6,307,892, perform bitwise addition modulo 2 of the codeword vector with a mask vector that is selected a priori, to be used with all codeword vectors, so as not to coincide with any of the possible codeword vectors. The method of Jones et al., and similar methods, are suboptimal in that they do not take into account the nature of the data actually being transmitted. For example, Jones et al. select a single mask vector to be used with all data.
The closest prior art solution to the present invention is that of Sharif and Hassibi, “Existence of codes with constant PMEPR and related design”, IEEE Transactions on Signal Processing vol. 52 no. 10 pp. 2836-2846 (October 2004). Given specific data to transmit, Sharif and Hassibi selectively change the signs of the symbols ξt to minimize PMEPR. Each symbol ξt is multiplied by the corresponding element εt of a balancing vector ε of length n, all of whose elements are either +1 or −1.
Both the Jones et al. patent and the paper by Sharif and Hassibi are incorporated by reference for all purposes as if fully set forth herein.
The solution proposed by Sharif and Hassibi has an unsatisfactorily large rate loss. For example, their method gives a zero rate for BPSK modulation and halves the transmission rate if QPSK modulation is used.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method of PMEPR reduction that would overcome the disadvantages of presently known methods as described above.